NuGet repositories: push does not handle SemVer 2.0 metadata correctly

[Zastai]

Reproduction steps:

• set up a hosted nuget repository
• configure it to disable redeploy
• push a Foo package with semver2 version 1.2.3+svn.420666.jenkins.68 (the metadata shows the SVN revision as well as the Jenkins build number)
• push a Foo package with semver2 version 1.2.3+svn.420666.jenkins.69

Expected result:

• the second push fails, because package Foo 1.2.3 already exists

Actual result:

• the push succeeds and the Nexus repo has both packages in it

We do not want builds for tags (whether triggered by accident, or because of build-technical adjustments that do not affect the package contents) to change the corresponding artifact (unless we explicitly go in and delete it from Nexus first).
A NuGet server implementation is supposed to ignore that metadata when determining the package ID (for purposes like dupe checks), but yours clearly does not.
(We also don't want such a retriggered job to fail, so the push uses --skip-duplicate, but it looks that is broken as well (#463).)

Note: our CD builds have that same info, but as part of the version info (so 1.2.3-pre.svn.420666.jenkins.69, because there we do want all of them in the repo.